The present invention relates generally to image transducing devices of the type wherein the image is stored as a charge pattern in a semiconducting target element having an array of diodes in one of its faces which is swept by an electron beam, and more particularly to a vidicon camera tube using such a target element.
The target element of the above type is basically comprised of a wafer of semiconducting material doped to have an N-type conductivity, with one of the major faces of the wafer being selectively doped to have a large plurality of P conductivity type regions, respective ones of the regions forming a junction diode with the substrate thereunder. The substrate is maintained at a potential which is positive with respect to the scanning electron beam so that, as the P conductivity type regions are bombarded with electrons they become reverse biased. In the reverse biased state each of the junction diodes stores an electric charge, electrons, derived from the beam and maintains that charge at least until it is scanned again. The customary scanning frequency is 30 hertz, making the time between successive scans 1/30 second.
The opposite face of the target element receives light from an image, and photons thus striking the target cause electron-hole pairs to be generated in the substrate of the target. A substantial number of the holes thus generated reach the diodes opposite the point of photon impact, where they combine with and hence eliminate a corresponding number of stored electrons. A charge pattern is thus created in the array of diodes, corresponding to the image striking the opposite face of the target element, with each diode having its stored charge diminished by an amount corresponding to the time integral of the light striking the corresponding spot on the light-receiving side of the target element. Consequently, upon its next scan, the electron beam charges each diode by an amount corresponding to the charge lost by that diode since it was last scanned, i.e., during the last 1/30 second. The amount of charging current from the electron beam and, more specifically, the photon-generated charges integrated by the target during each scan cycle, are detected by circuitry associated with the target to provide a signal representative of the detected image.
One limitation of existing vidicons of the above type is that, unlike earlier vidicons having an Sb.sub.2 S.sub.3 target, the sensitivity or gain of the silicon diode array vidicon cannot be varied by means of the applied target voltage. The reason for this is that the ratio of electronic charges in the output to abosorbed photons at the input is independent of target voltage over its useful range. In short, regardless of what the target voltage is within its useful range, diodes lose their charge at the same rate for an impinging image of given brightness. This places a limit on just how bright an image the devices can detect, since each diode can receive and store only a finite amount of charge from the scanning electron beam. If any point in the image is bright enough to deplete of its charge the diode at which it is projected, and to do so before the diode is scanned again by the electron beam, that point in the image will not be faithfully represented. Thus, silicon vidicon junction diode targets impose a severe limit on available dynamic range.
In order to prevent the dynamic range of the tube from being exceeded, it has been necessary to control the number of photons reaching the target by means of filters, or by means of a lens aperture. Such optical or mechanical devices have obvious disadvantages and various methods have been proposed for controlling sensitivity electronically. None of the techniques developed to date have been free of problems, however. These include a shift of spectral response and a sacrifice of resolution due to modifications in the target structure.